Porous carbon materials (CMs) with high specific surface area are widely used for many electrochemical, catalytic, and adsorption applications. The preparation of such materials usually involves two steps: 1) formation of carbon via carbonization of precursor material; 2) activation of carbon in order to enhance its surface area.
Porous CMs may be produced via carbonization of naturally occurring raw materials, such as wood, petroleum pitch, peat, and other sources of high carbon content. A major advantage of CMs prepared from naturally occurring raw materials is their relatively low cost. At the same time, such CMs contain large amounts of impurities such as sulfur, nitrogen, phosphor and metal salts, which initially reside in the precursor material. Such impurities may introduce undesirable side reactions when carbon material is employed, for example, in energy storage devices such as lithium-ion batteries, fuel cells or double layer capacitors. These side reactions may deteriorate the structure and lower the performance of the device.
Porous CMs may also be formed by carbonizing synthetic materials of high carbon content, for example polymers, at very high temperatures in a non-oxidative (inert) atmosphere, for example nitrogen, argon, or helium. The most widely-employed synthetic polymer precursor for making CMs is polyacrylonitrile. Other precursors such as phenolic resin and polyacetylenes may also be used. The disadvantage of CMs made from synthetic polymers is that these CMs have a very low specific surface area.
To enhance the surface area of CMs, activation is always performed after the carbonization process. The physical activation is accomplished with steam, carbon monoxide (CO), carbon dioxide (CO2), and CO2-containing gases. The chemical activation agents are ZnCl2, H2SO4, H3PO4, NaOH, LiOH, KOH, NxOy [x=1-2, y=1-3], Cl2 and other halogens. Activation provides for enhanced surface area of CMs, but it can introduce defects or completely destroy a formed carbon body.
To be useful for electrochemical and catalytic applications, the resulting high surface area CM should have the following properties: nano- or molecular-level organization of carbon structure, with structural elements (carbon fragments) ranging in size from 1 nm (molecular dimensions) to 10-100 nm (nano dimensions); regulated distribution of structural elements (carbon fragments), that can be adjusted to suit the material application; narrow pore size distribution; high electronic conductivity; high chemical stability and mechanical strength; and low cost.
A major problem in many carbon material applications is a relatively high internal resistance of CMs, which can be lowered through use of metal-carbon materials (MCMs). Uniform imbedding of metal atoms in the form of, for example, nano-sized particles or clusters into the porous carbon structure also improves and expands the catalytic properties of the discussed MCMs.
Various fabrication techniques for preparation of metal-carbon materials have been disclosed. One of the methods of fabrication implies thermal catalytic decomposition of hydrocarbons in the microporous metal matrix. One method describes the synthesis of MCM by thermal chemical vapor deposition of ethane in the presence of hydrogen at 660° C. on sintered metal fiber filters of nickel and Ni-containing alloys.
Another technique implies impregnation of high surface area porous carbon with metal precursors (metal salts or metal complexes) followed by their reduction to pure metals or metal oxides. For example, one method describes the approach where the carbon fiber material is dipped into an aqueous solution of ruthenium chloride followed by thermal decomposition to ruthenium oxide formed in the pores of the carbon fibers.
The majority of fabricated metal-carbon materials have metal-carbon globules or fibers of different pre-defined dimensions, but the relative distribution of globules (or fibers) is chaotic and difficult to regulate. This difficulty in regulation prevents the preparation of MCMs with pre-defined and controllable properties that suit the material application, which in turn limits the widespread use of these materials.
The present invention addresses this need.